// 动态规划
const maxProfit = prices => {
    const len = prices.length;
    const dp = new Array(len).fill(0).map(x => new Array(5).fill(0));
    dp[0][1] = -prices[0];
    dp[0][3] = -prices[0];
    for (let i = 1; i < len; i++) {
        dp[i][0] = dp[i - 1][0];
        dp[i][1] = Math.max(dp[i - 1][1], dp[i - 1][0] - prices[i]);
        dp[i][2] = Math.max(dp[i - 1][2], dp[i - 1][1] + prices[i]);
        dp[i][3] = Math.max(dp[i - 1][3], dp[i - 1][2] - prices[i]);
        dp[i][4] = Math.max(dp[i - 1][4], dp[i - 1][3] + prices[i]);
    }
    return dp[len - 1][4];
};